1. Field of the Invention
The present invention generally relates to heads-up display devices, and more particularly to a display using a real image projection system and a polarizing reflector element as a non-blocking mirror in heads-up display systems.
2. Background Information
Helmet mounted displays and windshield mounted displays (hereafter referred to as heads-up displays or HUDs) are increasing in importance in both military and civilian environments. For example, in a military setting, a heads-up display presents valuable information to pilots by superposing virtual images of data information over the pilots' normal field of vision usually focused at infinity. The flight parameters of the aircraft, navigational displays or any other relevant information may be viewed without the pilot moving his/her head or changing the focusing distance, thereby generally improving flight performance and safety. HUDs are also finding wider usage in civilian transportation, with windshield displays becoming popular in many automobiles. Further, HUDs are beginning to find a wide range of application in the entertainment markets, such as in videos, video games, and virtual reality. As used herein, the term “HUD” shall refer to a display which superimposes visual information (usually data, but may also be natural scenes) on the normal ambient scene.
Current information or image providing systems not based on HUD require drivers to steer their eyes away from the primary scene (e.g., highway) in order to read the information positioned away from the windshield. This mode of operation is highly unsafe. Using a HUD is inherently safer since the driver can keep looking forward and read the information simultaneously. This advantage was recognized, for example, in airplanes where a pilot, in certain circumstances cannot afford not to look forward. However, current devices suffer low light efficiency. For vehicular applications HUD systems are also required to be inherently compact and have high brightness. Another critical requirement of automotive HUD systems is low cost, and hence cannot use the same expensive parts as in airplanes.
The fundamental operation of HUDs is based on known projection of an image onto a reflective surface also known in general as a “combiner”. For example, Berman U.S. Pat. No. 4,900,133 describes an HUD system whereby a CRT or LCD is used to generate an image to be displayed, and a circular polarizing filter is used as combiner to direct light having a bandwidth in the reflective range of the combiner toward the viewers' eyes. Berman described one example whereby both left handed and right handed polarized light are directed to the combiner, and another whereby only one handedness is directed to the combiner. In the embodiment whereby both left handed and right handed polarized light are directed to the combiner, a limitation exists in that ambient light from outside in the selected bandwidth of is essentially blocked. In the embodiment where one handedness is used and a CRT display is provided, a polarizer is used to filter out the non-selected handedness. This results in low light efficiency, since the polarizing filter reduces at least 50% of the light. Further, where LCD displays are used, thermal management becomes a problem.
In another system, described by MicroVision (e.g., see http://www.mvis.com/prod_auto_hud.htm_and http://www.mvis.com/pdfs/sid_auto.pdf), a blocking screen is used to project an image at a lower portion of the windshield, thus prohibiting the driver to see through the blocking portion from inside of the car. This system is lacking in that the driver may not view ambient conditions simultaneously with the image display.
In still a further system proposed by MicroVision, a laser scanner is used as the image source. The laser scanner (e.g., under the dashboard) outputs polarized light. The polarized light within a characteristic bandwidth is directly reflected from a polarizing reflector to the user's line of sight. However, such a system results in an image that appears to float in front of the driver, as opposed to being viewable at the windshield itself. The display is directly imaged onto viewer's retina, which causes discomfort and strict positioning of driver's head.
Thus, known HUD systems lack high light efficiency, haze-free operation, high transmissivity (of ambient light) and high reflectivity (of information image) projection system for HUD applications. Accordingly, a need exists for a simple and inexpensive image generating system for use within a HUD systems that provides a bright image display and minimizes or eliminates detriments associated with the blocking or partial blocking of ambient view.